Objective
To examine whether ceiling effects at long inter beat intervals (IBIs)cause an underestimation of cardiac vagal control in regular exercisers by time and frequency-domain measures of respiratory sinus arrhythmia (RSA).
Methods
24-hour ECG and respiration recordings were performed in 26 regularly exercising subjects, actively engaged in aerobic training for the past year, and enrolled in supervised training in the six weeks pre-study, and in 26 age- and sex-matched non-exercisers. Sleep and waking levels of cardiac vagal control were estimated by RSA obtained through the peak–valley method, by the standard deviation of the IBIs, the root mean square of successive IBIs, and the high frequency IBI spectral power.
Results
In 11 of the exercisers the IBI–RSA relationship was characterized by a quadratic relationship. This reflected a ceiling effect at very long IBI values attained by regular exercisers, particularly during the nighttime recording. Irrespective of this ceiling effect, RSA as well as other heart rate variability (HRV) measures was still significantly larger in the exercisers with a quadratic IBI–RSA relationship than in non-exercisers or exercisers with a linear IBI–RSA relationship.
Conclusions
We conclude that a subgroup of regular exercisers is characterized by a low heart rate paired to high levels of cardiac vagal control. In these exercisers, vagal control is underestimated from HRV measures in ambulatory recordings. Inspection of the IBI–RSA relationship should be routinely added when HRV measures are used to index cardiac vagal control.

Physical activity, alcohol use and smoking might affect cardiovascular disease through modifying autonomic nervous system (ANS) activity. We investigated: 1) whether there are consistent relationships between lifestyle factors and cardiac ANS activity over time, and 2) whether 2-year changes in lifestyle factors relate to 2-year changes in cardiac activity. Baseline (n=2618) and 2-year follow-up (n=2010) data of the Netherlands Study of Depression and Anxiety was combined. Baseline data was collected in the Netherlands from 2004–2007. Lifestyle factors were habitual physical activity, frequency of sport activities, alcohol use, and smoking. Indicators of cardiac activity were heart rate (HR), respiratory sinus arrhythmia (RSA) and pre-ejection period (PEP) (100min of registration). The results showed that high physical activity (−1.8beats/min compared to low activity), high frequency of sport activities (‘couple of times/week’: −2.5beats/min compared to ‘almost never’) and mild/moderate alcohol use (−1.2beats/min compared to non-drinking) were related to low HR. Heavy smoking was related to high HR (>30cigarettes/day: +5.1beats/min compared to non-smoking). High frequency of sport activities was associated with high RSA (‘couple of times/week’:+1.7ms compared to ‘almost never’) and moderate smoking with longer PEP (11–20cigarettes/day: +2.8ms compared to non-smoking). Associations were consistent across waves. Furthermore, 2-year change in frequency of sport activities and number of smoked cigarettes/day was accompanied by 2-year change in HR (β=−0.076 and β=0.101, respectively) and RSA (β=0.046 and β=−0.040, respectively). Our findings support consistent effects of lifestyle on HR and parasympathetic activity in the expected direction. Cardiac autonomic dysregulation may be partly mediating the relationship between lifestyle and subsequent cardiovascular health.

The cross-stressor adaptation hypothesis—which posits that adjustment to physical stress as a result of regular physical activity and its effects on fitness crosses over to psychological stress reactivity—has been around for over four decades. However, the literature has been plagued by heterogeneities preventing definitive conclusions. We address these heterogeneity issues in a combined laboratory and daily life study of 116 young adults (M = 22.48 SD = 3.56, 57.76% female). The exposure, i.e., the potential driver of adaptation, was defined in three ways. First, a submaximal test was performed to obtain aerobic fitness measured as the VO2max (kg/ml/min). Second, leisure time exercise behavior, and third, overall moderate-to-vigorous physical activity (MVPA), were obtained from a structured interview. Outcomes were autonomic nervous system (ANS) reactivity and affective responsiveness to stressors. ANS activity was measured continuously and expressed as inter-beat-interval (IBI), pre-ejection-period (PEP), respiratory sinus arrythmia (RSA), and non-specific Skin Conductance Responses (ns.SCR). Negative and positive affect were recorded after each experimental condition in the laboratory and hourly in daily life with a nine-item digital questionnaire. Linear regressions were performed between the three exposure measures as predictors and the various laboratory and daily life stress measurements as outcomes. Our results support the resting heart rate reducing effect of aerobic fitness and total MVPA in both the laboratory and daily life. We did not find evidence for the cross-stressor adaptation hypothesis, irrespective of ANS or affective outcome measure or whether the exposure was defined as exercise/MVPA or aerobic fitness.

Selecting a valid baseline for measuring cardiac autonomic regulation is essential for psychophysiological research. Current guidelines typically recommend eyes-closed resting baselines, preceded by 5 min with eyes open. However, baseline autonomic regulation may vary as a function of both sensory-attentional state (i.e., eye state) and interindividual factors such as physical activity (PA). The present study examined differences between eye-states in autonomic regulation and respiration across a standardized baseline protocol (eyes open [EO1], eyes closed [EC], eyes open [EO2]) and investigated whether these responses are associated with habitual and leisure-time PA. In total, N = 100 university students were recorded during three consecutive 5-min sessions using electrocardiography and impedance cardiography (VU-AMS). Heart rate (HR), root mean square of successive differences (RMSSD), high-frequency power (HF), and respiration rate (estimated via impedance cardiography) were examined. Physical activity was assessed via questionnaires. Mixed-effects models tested condition effects and PA associations, including interactions. Eye state and temporal order significantly modulated cardiac activity, with higher HR and lower vagally mediated HRV during EC and EO2 compared to EO1. Respiration rate differed between EC and subsequent EO2. Low- and moderate-intensity leisure-time PA showed negative associations with vagal indices and interacted with both eyes-open conditions to predict RMSSD. The fixed EO1–EC–EO2 protocol provides reliable baseline estimates consistent with the Vagal Tank Theory and Neurovisceral Integration Model. As the first empirical evaluation of this baseline sequence, the study advances HRV methodology, clarifies context-dependent PA–HRV relationships, and offers normative autonomic data that strengthen experimental and applied psychophysiology.